Thermal analysis of spontaneous combustion behavior of partially oxidized coal

Process Safety and Environmental Protection 1 0 4 ( 2 0 1 6 ) 218–224

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Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep

Thermal analysis of spontaneous combustion behavior of partially oxidized coal Jun Deng a,b , Jingyu Zhao a,b,∗ , Yanni Zhang a,b , Anchi Huang c , Xiangrong Liu a,d , Xiaowei Zhai a,b , Caiping Wang a,b a

Key Laboratory of Western Mine Exploitation and Hazard Prevention of Ministry of Education, Xi’an, 710054 Shaanxi, PR China b School of Safety Science and Engineering, Xi’an University of Science and Technology, Xi’an, 710054 Shaanxi, PR China c Graduate School of Engineering Science and Technology, National Yunlin University of Science and Technology, Douliou, Yunlin 64002, Taiwan, ROC d School of Chemistry, Xi’an University of Science and Technology, Xi’an, 710054 Shaanxi, PR China

a r t i c l e

i n f o

a b s t r a c t

Article history:

Research on partially oxidized coal helps in the early detection of spontaneous combus-

Received 7 August 2015

tion due to secondary oxidation of the coal remaining in mined-out areas. Three types of

Received in revised form 29 August

coal samples were used in this study. A self-designed temperature-programmable experi-

2016

mental system was developed to investigate the spontaneous combustion characteristics

Accepted 6 September 2016

of partially oxidized coal. In addition, the differences between the oxidation characteristics

Available online 14 September 2016

of a fresh and a partially oxidized coal sample were studied. The CO concentration and its production rate, the oxygen consumption rate and the heat release rate were used as

Keywords:

macro-characteristic parameters to show the oxidation characteristics. The results show

Spontaneous combustion

that the characteristic parameters increased faster with temperature for partially oxidized

Partially oxidized coal

coal compared to a fresh sample up to a temperature of 110–140 ◦ C. Above this temperature

CO concentration and production

the opposite is observed. © 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

rate Oxygen consumption rate Heat release rate Macro-characteristic parameters

Contents 1. 2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 2.1. Coal samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 2.2. Surface characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 2.3. Oxidation experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 2.4. Indicator gas analysis method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 3.1. Surface characteristics of coal samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 3.2. CO concentration and characteristic temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

∗ Corresponding author at: Key Laboratory of Western Mine Exploitation and Hazard Prevention of Ministry of Education; School of Safety Science and Engineering, Xi’an University of Science and Technology, Xi’an, Shaanxi, 710054, PR China. Tel.: +86 186 2942 6155. E-mail address: [email protected] (J. Zhao). http://dx.doi.org/10.1016/j.psep.2016.09.007 0957-5820/© 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Process Safety and Environmental Protection 1 0 4 ( 2 0 1 6 ) 218–224

4.

1.

219

3.3. CO production rate and oxygen consumption rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 3.4. Heat release rate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .222 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224

Introduction

Coal is one of the most crucial energy resources worldwide (Diniz Da Costa et al., 2004; Warmuzinski, 2008). In China, the frequency of a fire caused by self-heating, or spontaneous combustion (GB/T 20104-2006, 2006) in a coal mine is higher than in other countries. Modern thermal experimental methods are adopted to examine the thermal behavior of spontaneous coal combustion (Qi et al., 2014, 2015; Bhoi et al., 2014). For instance thermogravimetry and differential scanning calorimetry (TG-DSC) are used worldwide to determine the oxidation behavior of coal (Saleh and Nugroho, 2013; Avila et al., 2014). Kinetic parameters, including activation energy, pre-exponential factor, and heat release rate, are important parameters to define the characteristics of spontaneous combustion (Li et al., 2014). Many experts adopted TG-DSC experiments to test the different factors which affect spontaneous coal combustion. Xu et al. (2013) studied the critical moisture content which affects the tendency of spontaneous combustion and found that there is a critical moisture content value, which is 20% for anthracite. Joshi et al. (2013) conducted TG tests with fresh and weathered dust coal samples. The activation energy and critical temperature indicated that the reactivity of weathered coal is lower than that of the fresh sample. Furthermore, a combination of TG and other experimental methods were adopted to verify spontaneous combustion features. Zhang et al. (2015a,b) combined TG and Fourier transform infrared spectroscopy (FTIR) to investigate the oxidation properties of lignite and bituminous coal. The kinetic parameters showed the lignite is easier to oxidize due to the lower activation energy than bituminous coal. The coal structure changed during oxidation indicating that aliphatic hydrogen groups, hydroxyls, and substituent groups in aromatic structures decreased, and carbonyl species increased in bituminous coal. In lignite, however, due to the formation of ether bonds, the carbonyls increased initially and then decreased. Properties of coal gangue were investigated by Sun et al. (2013) through TG-DSC, X-ray fluorescence, and X-ray diffraction. During the build-up to self-ignition, the structure of coal gangue is destroyed to a larger extent than fresh coal, more amorphous material is formed, the crystallization rate is lower, the distribution of elements is more dispersive, and the activation energy for oxidation are higher. During mining the mine face and the lower part of the thick coal seam (or coal seam group), oxidation of residual coal can readily occur in mined-out areas, and hence, the risk of spontaneous combustion is more severe (Bowes, 1984; Moghtaderi et al., 2000; Perdochova et al., 2015). Initial oxidation is a process in which fresh coal is partially oxidized during mining, storage, transportation and other conditions. When the partially oxidized coal oxidizes again, a so-called secondary oxidation process takes place. At present, experimental investigations into secondary oxidation concentrate on macro- and micro-characteristics (Jia et al., 2000; Manina et al., 2012; Xia et al., 2015; Uslu et al., 2012). Macroexperiments focus mainly on CO and CO2 production, oxygen

consumption rate, heat release rate, and the limiting parameters (i.e. minimum oxygen concentration, minimum thickness of residual coal, and maximum air leakage intensity) of secondary oxidation (Miroshnichenko and Desna, 2014). Gas production (i.e. CO and CO2 ) at low temperature (<170 ◦ C) oxidation is of significance for determining the potential extent of the fire. Baris et al. (2012) and Zhang et al. (2015a,b) conducted self-designed oxidation experiments to test CO and CO2 production and found that CO and CO2 production were sensitive to temperature. Finally, an effective method was developed to control spontaneous combustion during the secondary oxidation of residual coal in mined-out areas. Several micro-analysis studies on coal molecule structure have examined spontaneous combustion during secondary oxidation (Grzybek et al., 2002; Geng et al., 2009; Petersen et al., 2008). Mahidin et al. (2002) used a proximate analyzer and FTIR to test fresh coal as well as partially oxidized coal from Indonesia, and found that the moisture and volatile matter content and the C–H and C=O groups content of partially oxidized coal is lower than that of fresh coal. It became apparent on the value macro-characteristic parameters (CO and CO2 production, oxygen consumption rate, et al) declined.

2.

Experimental

2.1.

Coal samples

A kind of long flame coal, non-caking coal, and weakly caking coal were used in this study. The elemental analyses results are shown in Table 1. Fresh coal samples that were not subjected to water injection, spraying, and other treatments were sealed and packed with multilayer plastic and nylon bags, and then transported to the laboratory. Each experiment was performed 5 times to ensure test reproducibility. The samples were prepared as follows. Sample I: A fresh coal sample was selected, pulverized, and screened. Next, 1000 g coal samples of mixed grain sizes, including 0–0.9, 0.9–3.0, 3.0–5.0, 5.0–7.0, and 7.0–10.0 mm per 200 g (GB 474-2008), were filled in sealed bags and placed in sealed containers. Sample II: This sample was prepared after the oxidation experiment, and the indicator gas analysis method was used to calculate the critical and cracking temperatures of the initial and secondary oxidations, respectively. A piece of fresh coal sample was pulverized to 0.075–0.105 mm (GB 474-2008), equally divided in 2 portions with one of them labeled as fresh coal sample. The remaining portion of fresh coal was placed in crucibles with a lid, and placed in the oven and subjected to low-temperature (<170 ◦ C) oxidation in air. When the temperature reached 170 ◦ C the temperature was fixed for 2 h. Then, the door of the oven was opened with the heating switched off and flushed with nitrogen until the temperature reached 30 ◦ C. The samples remained in the oven during this procedure. After fixing the temperature at 30 ◦ C for 1 day with the opened door, the sample was taken from the oven and labeled partially oxidized coal.

220

Process Safety and Environmental Protection 1 0 4 ( 2 0 1 6 ) 218–224

Table 1 – Proximate and elemental analysis of coal. Samples

Long-flame coal Non-caking coal Weakly caking coal

C

H

O

N

Mad

Aad

Vad

1-Y

2-Y

1-Y

2-Y

1-Y

2-Y

1-Y

2-Y

1-Y

2-Y

1-Y

2-Y

1-Y

2-Y

74.78 76.17 79.69

71.73 71.51 72.89

4.84 4.52 4.96

4.53 3.77 4.89

19.92 17.85 13.30

21.80 22.03 19.57

1.25 1.02 1.45

1.18 0.74 1.56

7.79 10.5 3.00

2.46 1.16 0.86

15.72 6.36 8.78

18.65 8.67 19.83

36.03 26.22 36.35

33.29 26.19 32.71

Fig. 1 – XK-I temperature-programmed experimental system and reactor.

2.2.

Surface characteristics 25000

2.3.

20000

CO concentration (ppm)

An autosorb-IQ-C Fully Automatic Adsorption Analyzer (Quantachrome, America) was used to obtain the surface characteristics of fresh coal and partially oxidized coal. Surface characteristics included the distribution of pore volume and specific surface area. The analyses were performed in a nitrogen environment. The samples were pretreated in liquid nitrogen for dehydration and deaeration.

Oxidation experiments

Spontaneous combustion of partially oxidized coal is ubiquitous in mining procedures. To evaluate the risk, a self-designed XK-I temperature-programmed experimental system (Xi’an University of Science and Technology, China) was adopted to test the oxidation characteristics of fresh and partially oxidized coal in low-temperature oxidation (<170 ◦ C). A series of experiments were conducted at low temperature (<170 ◦ C) to obtained the characteristic parameters (CO concentration, CO production rate, oxygen consumption rate, et al.). The results would provide indications for prevention and control of spontaneous combustion of partially oxidized coal. The XK-I temperature-programmed experimental system is composed of five parts (Fig. 1(a)): an experimental oven; a reactor; a gas cylinder; a water trap; and a SP-3430 gas chromatograph analysis system. As Fig. 1(b) shows, the reactor is mounted vertically in the middle of the oven. The diameter was 9.5 cm, the length was 25.0 cm, and the coal loading capacity was up to 1.0 kg. Approximately 2.0 cm of free space was left at the upper and lower ends of the reactor. The free space at the bottom was made possible by introducing copper wire mesh to hold the coal. Air (21% O2 ) was used as the reaction gas during the experiment. A thermocouple was inserted into the reactor charged with the coal sample for monitoring the temperature of coal in the reactor. The airflow rate through the charged reactor was 120 mL/min, and the temperature was raised from 30 to 170 ◦ C at a rate of 0.3 ◦ C/min. The experimental error of the SP-3430 gas chromatograph is ±1% and of the temperature-programmed system less than ±2 ◦ C.

Critical temperature

15000

Crack temperature

10000

5000

0 0

20

40

60

80

100

120

140

160

180

Temperature (oC)

Fig. 2 – Testing characteristic temperatures by indicator gas analysis method.

2.4.

Indicator gas analysis method

The indicator gas analysis method refers to selecting some appropriate gases whose concentrations vary in accordance with coal temperature. Detectability, high sensitivity and stability are the necessary features of indicator gases. The concentrations of indicator gases manifest themselves in a particular fashion as a function of the temperature of the coal samples. Carbon monoxide is an ideal indicator gas for the prediction of spontaneous coal combustion as it is the major gas emission in the low-temperature oxidation range (Zhang et al., 2013; Baris et al., 2012; Tang, 2015). Its concentration represents an exponential relationship with coal temperature as shown in Fig. 2. The characteristic temperatures of coal oxidation include the critical temperature and cracking temperature in low temperature oxidation (<170 ◦ C). The critical temperature is the key characteristic during coal spontaneous combustion, and is the transition point from low to rapid oxidation. As shown in the Fig. 2, the critical temperature is the first point of acceleration in the CO concentration curve of the coal–oxygen reaction. Furthermore, the intersection between the tangent

221

Process Safety and Environmental Protection 1 0 4 ( 2 0 1 6 ) 218–224

Table 2 – Distribution of pore volume. Hole diameter

Micropore (%)

Long-flame coal 1-Y Long-flame coal 2-Y Non-caking coal 1-Y Non-caking coal 2-Y Weakly caking coal 1-Y Weakly caking coal 2-Y

Medium pore (%)

1 73 27 74 6 89

77 18 56 19 77 8

3.2.

Table 3 – Specific surface area of coal. 1-Y/m2 g−1

Coal samples Long-flame coal 1-Y Non-caking coal 1-Y Weakly caking coal 1-Y

2-Y/m2 g−1

3.143 8.721 3.086

of the critical temperature and maximum tangent of the CO production rate is the cracking temperature.

Results and discussion

3.1.

Surface characteristics of coal samples

Coal sample II was chosen for analysis of the pore size and specific surface area. The results are listed in Table 2. Coal is not a homogeneous substance, and it includes cracks, joints, fusinite cell cavities, and other pores. Based on their size, the pores can be divided into three types: large, medium, and micro. The diameter of a large pore is more than 100 nm, that of a medium pore is 10–100 nm, and that of a micropore is less than 10 nm (Gürdal et al., 2015). In the fresh coal sample, the number of medium pores was highest. The structure of partially oxidized coal was altered due to oxidation, and thus, more micropores were present. Furthermore, the number of medium and large pores decreased. Therefore, the average pore size of partially oxidized coal was smaller than that of fresh coal. The specific surface area of partially oxidized coal was eight times larger compared to fresh long-flame coal (Table 3). After initial oxidation, fresh coal tended to crumble, and its porosity increased, leading to an increase in specific surface area. The probability of partially oxidized coal to react with oxygen therefore increases, consuming more oxygen, generating more CO, and releasing more heat than fresh coal.

CO concentration and characteristic temperatures

3.3.

CO production rate and oxygen consumption rate

The CO production rate and oxygen consumption (Nordon et al., 1979) are crucial characteristic parameters of spontaneous combustion. The CO production rate was calculated as follows (Xu, 2001): dCco = Vco (T)d,

d =

dz , u

u=

25000

20000

20000

20000

Initial oxidation

10000

5000

15000

Initial oxidation

10000

Secondary oxidation

5000

Second oxidation 0

0 0

20

40

60

80

100

120

140

160

180

CO concentration (ppm)

25000

15000

15000

10000

5000

Second oxidation

0 0

20

40

60

80

100

120

140

160

180

0

o

(a) Relationship between CO concentration and temperature of long-flame coal

(1)

Initial oxidation

Temperture ( C)

Temperature (oC)

Q Sn

where Cco is the CO concentration (ppm), Vco (T) is the CO production rate (mol/cm3 s), z is the distance to the air intake (cm), u is the average velocity of air in the void (cm/s), Q is the air

25000

C O concentration (ppm )

CO concentration (ppm )

22 9 17 7 17 3

Sample I was used for the experiment. The characteristic parameters including CO gas concentration, characteristic temperatures, oxygen consumption rate (Nugroho et al., 2000), and heat release rate were determined. Typically CO, CO2 , and lower alkanes, alkenes and alkynes were released during oxidation. Among them, CO was the most pronounced emission during oxidation (García-Torrenta et al., 2012; Fauconnier, 1992). Fig. 3 shows that the CO concentration of the coal samples increased with temperature. The CO concentration at secondary oxidation was higher than that from initial oxidation at lower temperatures (30–140 ◦ C). As the temperature increased, the CO concentration emanating from secondary oxidation increased less steeply than that from the initial oxidation. Based on the indicator gas analysis method, the critical and cracking temperatures were obtained from the CO concentration curve, see Table 4. As can be seen from this table, the characteristic temperatures of partially oxidized coal were lower than those of fresh coal.

26.535 45.342 13.081

3.

Large pore (%)

(b) Relationship between CO concentration and temperature of non-caking coal

20

40

60

80

100 120 140 160 180 o

Temperture ( C)

(c) Relationship between CO concentration and temperature of weakly caking coal

Fig. 3 – Relationship between CO concentration and temperature.

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Process Safety and Environmental Protection 1 0 4 ( 2 0 1 6 ) 218–224

Table 4 – Characteristic temperatures of coal. Critical temperature (◦ C)

Coal sample

Crack temperature (◦ C)

Initial oxidation

Secondary oxidation

Initial oxidation

Secondary oxidation

70 80 80

50 60 70

110 120 120

100 100 110

Long-flame coal Non-caking coal Weakly caking coal

flow (cm3 /s), S is the oven cross sectional area (cm2 ), and n is voidage (%). The CO and oxygen concentrations at a certain point zi are denoted as Cico (ppm) and Ci (ppm), respectively; C0 is the standard oxygen concentration (ppm); V0 (T) is the oxy0 (T) is gen consumption rate of coal in air (mol/cm3 s); and Vco 3 the standard CO production rate (mol/cm s). Equation (2) can thus be derived as follows:



z2

C2co − C1co = z1

=

Vco (T) dz = u

0 (T) nSVco



QC0

z2



z2

z1

C1 e−

The oxygen consumption rate is proportional to the oxygen concentration; thus, Eq. (5) can be written as follows: dC = −V0 (T) ×

0 (T) S CVco ndz Q C0

z1

Therefore, based on Eq. (2), the standard CO production rate is expressed as follows:

0 Vco (T) =

V0 (T)(C2co − C1co )

(3)

C1 [1 − e−V0 (T)s(z2 −z1 )/QC0 ]

The CO production rate as a function of temperature for the different coals is shown in Fig. 4. The oxygen consumption rate [Eq. (4)] can be obtained as follows: dC = −V(T), d

d =

dx Q¯

(4)

where V(T) is the oxygen consumption rate (mol/m3 s),  is the reaction time (s), and Q¯ is average air flow of a plane with coal in the reactor (cm3 /s). Hence, ¯ QdC = −V(T) dx

QC0 C1 ln C2 S(z2 − z1 )

(7)

The oxygen consumption rate as a function of temperature for the different coals is given in Fig. 5. Figs. 4 and 5 show as the temperature increases, the CO production rate and the oxygen consumption rate readily increased. As for long-flame coal, the CO production and the oxygen consumption rate of the secondary oxidation samples were larger than those of initial oxidation up to 110 ◦ C. With the temperature exceeding 110 ◦ C, those two rates for the secondary oxidation samples were lower than those of initial oxidation. Similarly, the noncaking coal and weakly caking coal had a higher rate of CO production and oxygen consumption for the secondary oxidation samples than those for the initial oxidation samples up to 140 and 100 ◦ C, respectively. The results reported in Table 5 show a greater consumption of oxygen and more generation of CO – and hence higher heat release rate – from partially oxidised coal compared to fresh coal. From Figs. 3–5 it can be seen that in the early stages (30–140 ◦ C) for the secondary oxidation samples the oxygen consumption rate, the CO gas production rate, the characteristic temperature, and the heat release rate were greater than for the corresponding initial oxidation samples.

(2)

dz

(6)

Integrating the two sides of Eq. (6) leads to Eq. (7): V0 (T) =

V0 (T)Sn (z−z1 ) QC0

C dx ¯ 0 QC

3.4.

Heat release rate

The heat release rate is a crucial factor of spontaneous combustion, reflecting the exothermicity of coal. The heat is

(5)

600

500 400

Initial oxidation

300 200 100

Secondary oxidation

0 0

20

40

60

80

100

120

140

160

180

Temperature (oC)

400

Initial oxidation 300

200

Secondary oxidation

100

0 0

20

40

60

80

100

120

140

160

180

CO production rate (10 11mol/cm 3 s)

500

CO production rate (10 11mol/cm 3 s)

CO production rate (10 11 m ol/cm 3 s)

600

500 400

Initial oxidation

300 200 100

Secondary oxidation

0 0

20

(a) Relationship between CO production rate and temperature of long-flame coal

(b) Relationship between CO production rate and temperature of non-caking coal

40

60

80

100

120

140

160

180

Temperature (oC)

Temperature ( oC)

(c) Relationship between CO production rate and temperature of weakly caking coal

Fig. 4 – Relationship between CO production rate and temperature.

223

4000

Initial oxidation

3000

2000

Secondary oxidation

1000

0 0

20

40

60

80

100

120

140

160

180

Oxygen consumption rate (10 11mol/cm 3 s)

Oxygen consumption rate (10 11mol/cm 3 s)

5000

5000

4000

Initial oxidation

3000

2000

Secondary oxidation

1000

0

0

20

40

60

80

100

120

140

160

(a) Relationship between oxygen consumption rate and temperature of long-flame coal

5000

4000

Initial oxidation

3000

2000

Secondary oxidation

1000

0

180

Temperature ( oC)

Temperature (oC)

Oxygen consumption rate (10 11 mol/cm 3 s)

Process Safety and Environmental Protection 1 0 4 ( 2 0 1 6 ) 218–224

(b) Relationship between oxygen consumption rate and temperature of non-caking coal

0

20

40

60

80

100

120

140

160

180

Temperature (oC)

(c) Relationship between oxygen consumption rate and temperature of weakly caking coal

Fig. 5 – Relationship between oxygen consumption rate and temperature. Table 5 – CO production rate and the oxygen consumption rate at break point during oxidation. CO production rate (mol/cm3 s)

Rate/Coal samples Long-flame coal Non-caking coal Weakly caking coal

Fresh coal at 110 ◦ C Partially oxidized coal at 110 ◦ C Fresh coal at 140 ◦ C Partially oxidized coal at 140 ◦ C Fresh coal at 100 ◦ C Partially oxidized coal at 100 ◦ C

5.2 × 1011 9.0 × 1011 120 × 1011 130 × 1011 5.9 × 1011 8.9 × 1011

derived from the coal–oxygen reaction. The actual heat release rate is between maximum and minimum value. As shown in Eqs. (8) and (9) (Wen et al., 2001): qmax (T) =

0 (T) Vco V0 (T)Hco 0 0 Vco (T) + Vco (T) 2

+

0 (T) Vco 2 V0 (T)Hco2 0 0 Vco (T) + Vco (T) 2

(8)

0 0 qmin (T) = Hr [V0 (T) − Vco (T) − Vco (T)] 2 0 0 + Hco Vco (T) + Hco2 Vco (T) 2

Oxygen consumption rate (mol/cm3 s)

(9)

where qmax (T) is the maximum heat release rate (J/cm3 s) and 0 (T) is the qmin (T) is minimum heat release rate (J/cm3 s); Vco 2

4.9 × 1013 6.1 × 1013 18 × 1013 22 × 1013 5.7 × 1013 6.7 × 1013

standard CO2 production rate (mol/cm3 s), Hco2 and Hco are the reaction heat of CO2 and CO (kJ/mol), respectively; and Hr = 58.8 kJ/mol (Xu, 2001). Based on the equations, the curves shown in Fig. 6 are obtained. As shown the maximum and minimum heat release rates of secondary oxidation coal exceeded those of initial oxidation at temperatures below 140 ◦ C. The heat release rate of secondary oxidation weakened gradually with an increase in temperature, and was less than that of initial oxidation above 140 ◦ C. At the same temperature, the heat release rate of coal was gradually attenuated. This is consistent with the change in the distribution of pore sizes which will allow the partially oxidized coal to more easily contact with oxygen. Thanks to the diminished pore size and increased specific surface area of partially oxidized coal, the contact area of coal is enlarged so that the coal–oxygen reaction is enhanced.

Fig. 6 – Relationship between heat release rate and temperature.

224

4.

Process Safety and Environmental Protection 1 0 4 ( 2 0 1 6 ) 218–224

Conclusions

A self-designed temperature-programmable experimental system was adopted to test the thermal characteristics of partially oxidized coal. This system was capable of testing the carbon monoxide concentration and production rate, oxygen consumption rate, and heat release rate. Partially oxidized coal was easier to react with oxygen than fresh coal. Compared with fresh coal, the carbon monoxide concentration and production rate, oxygen consumption rate and heat release rate of partially oxidized coal were higher at temperatures up to 140 ◦ C. It was inferred that as the surface area increases and micropores constitute the major percentage of coals in partially oxidized coal. This results in an increase in the contact area for the coal–oxygen reaction.

Acknowledgments The project supported by National Natural Science Foundation of China (Grant No. 51134019 and 51504187), Natural Science Basic Research Plan in Shaanxi Province of China (Program No. 2016JM5016).

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